Wireless communication between electronic devices may be accomplished using a wide variety of communication media, communication systems and communication standards. For example, portable electronic devices such as mobile telephones are typically configured to communicate via analog and/or digital wireless radio frequency (RF) telephone systems. Such devices may additionally be configured to communicate using wired and/or wireless local area networks (LANs), short range communication channels such as Bluetooth RF communication channels and/or infrared communication channels, and/or long range communication systems such as satellite communication systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly referred to as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity. Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
In any communication system, a user may desire to send a message to an intended user without the message being read or altered by an unintended third party. Some communication media are inherently more secure than others. For example, communication media that are not under the control of the sender and receiver may be considered physically insecure, since a third party may eavesdrop on communications sent over the media. Some examples of such physically insecure communication media are RF and other non-directional wireless communication systems. In such systems, an eavesdropper having an antenna within range of the transmitter may intercept messages that were intended for another receiver without the sender realizing the message was received by the eavesdropper. The internet is another example of a physically insecure communication system, since messages sent over the internet may pass through uncontrolled network nodes, and may thus be viewable by anyone having access to the node.
In order to overcome the limitations of such systems, it is known to encrypt and decrypt messages using, for example a secret key, so that even if the message is intercepted by a third party, the message may not be understandable by the third party. A secret key that is known to both the sender and the receiver of the message is commonly referred to as a “symmetric” key. Symmetric key encryption systems may also be referred to as “private key” encryption systems.
In contrast to private key encryption systems, public key (or “asymmetric key”) encryption systems use a public key to encrypt data and a private key, ostensibly known only to the recipient of the encrypted data, to decrypt the data. In a public key encryption system, data encrypted with a public key can generally only be decrypted with the corresponding private key. Likewise, data encrypted with a private key can only be decrypted with the corresponding public key. Such a feature is commonly used for authentication purposes, e.g. electronic signatures.
A number of key encryption algorithms, including public key encryption algorithms, have been developed. While potentially useful, such algorithms may be subject to attack by malicious parties. In addition, public key encryption algorithms may also be computationally expensive and may require the involvement of third party certificate authorities to ensure the authenticity of shared public keys.
In contrast to the physically insecure communication media described above, some communication channels may be inherently physically secure. For example, in point-to-point channels in which access to the communication media is physically secured, messages may be exchanged as clear text without significant risk of unwanted interception. The exclusive use of such systems may be severely limiting, however, since by definition such systems do not have the flexibility associated with communicating over widely distributed networks such as wireless RF networks and/or the internet. In many cases, and in particular in a mobile environment, a user may desire to send a message using a communication channel that is, at least in part, physically insecure. Moreover, even with data encryption, it will be appreciated that varying levels of encryption may provided which may result in varying levels of effective security. For example, some encryption protocols may use longer keys (e.g. 128 bits), while other protocols may use shorter keys (e.g. 64 bits).